Which style alternator do I need?
Define "correct" If you want it to look like a 69 you need a 69/ earlier so called "round back." If you can stand the look and want better low RPM performance, buy the later so called "square back."
Now "round backs" were also used for the first few 70-71?-72? until the squareback came out, BUT Mopar added a second brush holder, so isolated field. You can use this, or a squareback, just ground one field terminal if you are using the 69/ earlier regulator
The caveat is, that many sleezy rebuilders MODIFY the older 69/ earlier alternators to convert them to isolated field (incorrectly called "dual field") What they do is to drill the case and install a second brush. These are INFERIOR
On the left, below, is a 70's "squareback." These only came in isolated field. On the right is an original 69/ earlier "roundback" with one grounded brush, at lower right between the two diodes at about 4-5 o'clock
Below is one of my favorite photos. This is an original 69/ earlier "roundback" which had been "hacked" by a sleezy rebuilder, and then converted BACK to grounded brush
The original isolated brush is at left at 9 o'clock
The original grounded brush, which is in place, is at the bottom at 6 o'clock
The "hacked" position for the second, added (missing) isolated brush is at the top at 12 o'clock
One REALLY big thing wrong with these is a --three things-----
Using a 'hacked' one on a mid 70's car gives reduced low RPM performance over an original squareback
The "hacked" brushes gave far more trouble, and often shorted, meaning if they were hooked to the regulator wire, they gave full unregulated output, and if hooked to the IGN wire, they often melted the ignition harness
AND you can easily find cases where they came "out of the box" from a sleezy rebuilder with ALL THREE brushes in place. This gives the same results as the paragraph above.
Define "correct" If you want it to look like a 69 you need a 69/ earlier so called "round back." If you can stand the look and want better low RPM performance, buy the later so called "square back."
Now "round backs" were also used for the first few 70-71?-72? until the squareback came out, BUT Mopar added a second brush holder, so isolated field. You can use this, or a squareback, just ground one field terminal if you are using the 69/ earlier regulator
The caveat is, that many sleezy rebuilders MODIFY the older 69/ earlier alternators to convert them to isolated field (incorrectly called "dual field") What they do is to drill the case and install a second brush. These are INFERIOR
On the left, below, is a 70's "squareback." These only came in isolated field. On the right is an original 69/ earlier "roundback" with one grounded brush, at lower right between the two diodes at about 4-5 o'clock
Below is one of my favorite photos. This is an original 69/ earlier "roundback" which had been "hacked" by a sleezy rebuilder, and then converted BACK to grounded brush
The original isolated brush is at left at 9 o'clock
The original grounded brush, which is in place, is at the bottom at 6 o'clock
The "hacked" position for the second, added (missing) isolated brush is at the top at 12 o'clock
One REALLY big thing wrong with these is a --three things-----
Using a 'hacked' one on a mid 70's car gives reduced low RPM performance over an original squareback
The "hacked" brushes gave far more trouble, and often shorted, meaning if they were hooked to the regulator wire, they gave full unregulated output, and if hooked to the IGN wire, they often melted the ignition harness
AND you can easily find cases where they came "out of the box" from a sleezy rebuilder with ALL THREE brushes in place. This gives the same results as the paragraph above.